I mean like whole coffee beans that have been ground up. I'm not even sure what you mean by soluble type. You mean like instant coffee? 'Cause that's stuff's kinda gross.
Yeah it is. But then the difference is not just brewing time... see, the water needs to be hot for the brewing to work, and when you put it in contact with the coffee, it cools down. Then double coffee would be stronger than coffee made with double the quantity of coffee, at least for that reason.
The other day me and my friend concocted a caffeine bomb at our institute's kitchen. We boiled the water, mixed it with the coffee powder, kept it on the fire, stirring as we went. THEN we had it go through the filter paper. Most effective means of extraction I've seen.